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Introduction In control theory, an on/off controller, also known as a hysteresis controller, is a feedback controller that switches abruptly between two states. These controllers may be realized in terms of any element that provides hysteresis. They are often used to control a…
Introduction Positioner is a device put into a valve to ensure that it is at correct position.An open/close operation of control valve also can be done with an I/P converter but the position of the valve can’t be confirmed. calibration of control…
PID (Proportional – Integral – Derivative ) Controller Proportional The proportional term makes the current error signal multiplied with a gain (Kp).The result will be output signal. So output signal = Kp * Error_Signal Integral The integral term makes the current error…
Override It is sometimes necessary to limit a variable in order to maintain safe operation or protect process equipment. If this variable is a function of the system’s primary controlled variable, the two variables can be interlocked in an override system. The…
Feedforward A Feedforward control system measures a disturbance in variable, predicts its effect on the process and applies corrective action. Given an exact model of the process, the feedforward controller will adjust the manipulated variable (m) so that the controlled variable (c)…
Cascade In cascade control system two controllers are cascaded to each other as shown in the diagram below. Primary controllers output is fed to secondary controller as a Set point. Both the controllers has their own sensing elements but only secondary…
Ratio Control In Ratio Control, the controlled variable is the ratio of two measured variables. Control is effected by adjusting one of the variables – the controlled variable, to follow in proportion to a second – the wild variable. The proportionality constant…
Common Control Loop 1.Flow Liquid flow processes are fast, typically 0.5 sec response or less. Gas flow is slightly slower because of the compressibility of the gas. Control components – transmitter, valve and transmission lines – are the main dynamic…
Derivative Control Derivative control (also called rate control) responds to the rate of change of the controlled variable. For example, if the temperature of a process is changing rapidly, the controller must respond quickly to keep the temperature within acceptable limits. In…
Integral (or Reset) Control Integral control (also called Reset control) can be used when unacceptable amounts of offset occur in a process with proportional control. Integral control is used to return the value of the controlled variable to the set…